The present invention relates generally to organic electroluminescent (EL) devices and more particularly to organic EL devices having a supplemental cathode bus conductor and contact structures formed over the cathode bus conductor which provide electrical contact between a light-transmissive cathode and the bus conductor.
Passive matrix organic EL devices are fabricated by sandwiching organic EL medium layers between patterned anodes and perpendicularly oriented cathodes. In a conventional pixelated passive matrix organic EL device, light-transmissive anodes, for example indium-tin-oxide (ITO) anodes, are formed on a light-transmissive substrate such as, for example, a glass substrate. Organic EL medium layers are deposited over the anodes and the substrate, and a cathode or cathodes are deposited over the EL medium layers.
Such conventional passive matrix organic EL devices are operated by applying an electrical potential (also referred to as a drive voltage) between an individual row (cathode) and an individual column (anode). When the cathode is biased negatively with respect to the anode, light emission results from a pixel defined by an overlap area of the cathode and the anode, and the emitted light reaches an observer through the anode and the substrate.
In order to display a message or an image with the conventional device, all rows (cathodes) must be actuated or addressed individually and within a frame time selected to be shorter than the response of the human visual system, so as to avoid the perception of a flickering display. Each individual row (cathode) is actuated for a fraction of the frame time (1/# of rows). Therefore, the pixels within a row must be operated or driven to provide a brightness of emitted light (luminance) which is a product of the number of cathode rows and an average value of the displayed luminance. Thus, a relatively high instantaneous luminance is required for each pixel in a row which, in turn, requires relatively thick (typically 0.15 to 0.3 micrometer) cathodes in order to conduct the drive current I to and from the cathodes without undue drop in drive voltage along a length dimension of a cathode. Such relatively thick cathodes are optically opaque and, therefore, preclude light emission through such cathodes.
Stated differently, if light emission through cathodes is desired in a passive matrix organic EL device, metallic cathodes must be sufficiently thin to allow for transmission of emitted light. However, as the cathode thickness is reduced, the cathode becomes unsuitable for conducting the required instantaneous drive current I because the resistance R of a cathode row increases with decreasing cathode thickness. Consequently, a voltage drop xcex94V=Ixc3x97R along a cathode increases, necessitating undesirably higher applied drive voltages.
Although the drawings shown in plan view depict schematically a passive matrix organic EL device or its precursor having four anodes and four cathodes, it will be appreciated that a relatively large-area high-resolution organic EL display panel will have a large number of cathode rows intersecting a large number of anode columns. In constructing such display panels, the cathode thickness has to be increased still further to conduct the instantaneous drive current I corresponding to the required instantaneous luminance of each pixel in a cathode row. Cathode thickness values of about 1 micrometer may be required to minimize an undesirable voltage drop
xcex94V=Ixc3x97R along each cathode of resistance R.
To provide effective cathode separation of such relatively thick cathodes requires relatively tall or relatively high cathode separation shadowing structures which are difficult to manufacture. Forming relatively thick cathodes has a further disadvantage in that minor defects in the organic EL medium layer, sandwiched between anodes and cathodes, can cause permanent xe2x80x9cshortsxe2x80x9d between an anode and a relatively thick cathode. Such xe2x80x9cshortsxe2x80x9d may be less pronounced and/or self-healing if relatively thin cathodes could be constructed.
It is therefore an object of the present invention to form a passive matrix pixelated organic EL device having cathodes of a cathode thickness which is too thin to carry a required instantaneous current, and to provide at least one electrical contact between each cathode and a corresponding cathode bus metal layer capable of carrying the required instantaneous current.
It is another object of the present invention to provide a method of making a passive matrix pixelated organic EL device having a cathode bus metal layer and at least one cathode bus shadowing structure formed over the cathode bus metal layer for providing electrical contact between a thin cathode and the cathode bus metal layer.
It is yet another object of the present invention to provide a method of making a passive matrix pixelated organic EL device having a plurality of spaced thin cathodes each of which is in electrical contact with a cathode bus metal layer which is in electrical contact with a cathode connector which extends inwardly from an edge of a device substrate.
It is a further object of the present invention to provide a method of making a passive matrix pixelated organic EL device having a plurality of spaced thin cathodes, each of which is in electrical contact with a cathode bus metal layer and wherein the cathode bus metal layer forms a cathode connector which extends to an edge of a device substrate.
It is another object of the present invention to provide a method of making a passive matrix pixelated organic EL device having a plurality of spaced light-transmissive cathodes, each of which is in electrical contact with a cathode bus metal layer and wherein the cathode bus metal layer forms a cathode connector which extends to an edge of a device substrate.
These and other objects and advantages are achieved in a method of making a passive matrix pixelated organic electroluminescent (EL) device having a thin cathode, comprising the steps of:
a) providing a substrate having a plurality of spaced anodes formed thereon and a plurality of spaced cathode connectors extending inwardly from an edge of the substrate for providing an electrical connection so that a drive voltage can be applied between a selected anode and a selected thin cathode to cause light emission from a pixel of the device formed by the selected anode and the selected cathode;
b) forming a plurality of spaced electrically insulative base layers over the anodes and the substrate which extend in a direction perpendicular to the anodes and over a portion of each of the spaced cathode connectors and forming an opening or a cut-out in the base layers to extend to the cathode connectors in the portion;
c) forming a conductive cathode bus metal layer over a portion of each of the base layers, the bus metal layer extending at least into the opening or cut-out to provide an electrical contact to each of the spaced cathode connectors;
d) forming an electrically insulative organic cathode separation shadowing structure over each of the base layers and forming at least one organic cathode bus shadowing structure over a portion of the cathode bus metal layer;
e) providing a mask defining a deposition zone over the substrate for depositing an organic EL medium layer and a conductive cathode over the organic EL medium layer;
f) first depositing the organic EL medium layer by vapor deposition of organic EL materials directed towards the substrate into the deposition zone and using a direction of vapor deposition of the organic EL materials with respect to the shadowing structures formed in step d) to cause formation of the organic EL medium layer to terminate at positions spaced from a base of each of the shadowing structures; and
g) second depositing a conductive thin cathode by a vapor deposition of conductive cathode materials directed towards the organic EL medium layer into the deposition zone and using a direction of vapor deposition of the conductive materials with respect to the shadowing structures formed in step d) to cause formation of a plurality of spaced thin cathodes, each of such spaced cathodes being in electrical contact with a corresponding cathode bus metal layer in the positions where the organic EL medium layer is spaced from the base of the at least one cathode bus shadowing structure.